The invention relates to a normal conductive or superconductive field coil comprising a plurality of concentric individual coils, mechanically and electrically separated from one another over their winding length.
Such field coils are particularly suitable as high field coils. A traditional solution of a high field coil is the so-called Bitter magnet in which slotted circular copper discs are stacked together with an insulating foil in such a way that a thick, single-layer helix is formed. In another known high field coil helically wound disc coils comprising a pretensioned strip of constant height and insulated, e.g. with a nylon band, are superimposed. It is also known to form individual wire-wound coils into a high field coil.
In these coils the individual coil parts or discs are secured to one another by screwing, casting or by similar attachment methods so that the strength of the thus formed coil former is generally sufficient to absorb the forces exerted by the electromagnetic fields determined by the Lorentz force density and the stresses caused by the fields, without there being any danger of damage to the coils or the coil former.
The Lorentz forces are dependent on the current density in the conductor. In the aforementioned coil types the current density distribution is constant over the coil radius. However, in Bitter magnets the radial current density distribution is inversely proportional to the radius, so that in the radially inner area of the coil there is a higher current density than in the radially outer area. Therefore in Bitter magnets the Lorentz forces proportionally dependent on the current density and consequently the mechanical stresses in the radially inner area of the coil are greater than in the radially outer area. As a result in Bitter magnets only those electromagnetic fields can be built up which in the radially inner area of the coil do not produce stresses which exceed the material-dependent tensile strength of the conductor material, whereby in this connection tangential and radial tensile stresses are of particular significance in the radially inner area. Therefore the size of the attainable fields is a strength problem but which, assuming a homogeneous hollow cylinder, has been solved with the aid of computers, so that distribution-like approximate solutions exist for the current density which reproduce the field distribution within the coils by a function.
As the Bitter magnet represents the conventional solution of a high field coil the prior art more particularly describes the further development of this high field coil type. It has been found that on the inner edge of the coil the radial tensile stress is much smaller than the tangential tensile stress and that the latter is significantly co-determined by the radial tensile stress. Thus, an effort is made to keep the radial tensile stress as small as possible, so as to also reduce the tangential tensile stress. This has been achieved with identical external dimensions by replacing one coil by two contacting concentric coils. Finally it has been found that the radial tensile stress is negligibly small if a very large number of thin individual coils are fitted into one another. In the extreme case, with an infinitely large number of coils of infinite thinness the radial tensile stress, independently of the radial current density distribution, is equal to zero and the tangential tensile stress can be determined on a radius r approximately as the product of the current density, the magnetic field and the radius.
These relationships substantially also apply to individual coils realizable with a finite thinness having a diameter ratio (external diameter to internal diameter) of e.g. 1:1. The tangential tensile stress is then only a few percent above the current density value resulting from the above relationship for the tangential tensile stress in coils of infinite thinness and is therefore lower to a factor of two or more than that of a traditional thick high field coil.
The measure of replacing a single thick coil by a plurality of concentrically nested, contacting or non-contacting coils is called mechanical separation. If these individual coils are also electrically insulated relative to one another by insulation, e.g. by a gap between them, reference is made to electrical separation. The latter leads to more favorable current density distributions, because it is possible to approximate stepwise virtually any desired radial current density distribution in that at the same current the wire dimensions or the superconducting part on the conductor is selected correspondingly for each individual coil. Thus, e.g. compared with a resistance magnet with constant current density the field produced is increased by 20% for the same external dimensions and same output if J.about.1/r.sup.2, J standing for the current density distribution and r for the radius of the coil.
In the case of superconducting coils account must also be taken of the fact that the maximum permitted current density is also dependent on the magnetic field, but here again the forces which occur, particularly in high field coils of Nb.sub.3 Sn or V.sub.3 Ga are important design parameters. In the case of a radial current density distribution guaranteeing the same maximum tangential tensile stresses in all individual coils volume economies of up to 50% are obtained compared with a single thick coil with constant current density, as a function of the field strength, dimensions and stability requirements.
As a result of the arrangement of a plurality of concentric individual coils separated mechanically and electrically from one another over their winding length the straining together or screwing of the individual coils to form a coil former is extremely difficult and complicated. Casting around would in fact eliminate the advantage of reducing forces. Thus, for example, in the case of a high field coil with mechanical separation of the individual coils it is known to secure the latter by a large number of bolts on their two faces in the axial direction. The bolts are guided and prestressed by a complicated and costly system of cylinders, pistons, rods, levers and rams. A radial fixing of the individual coils, as is necessary for an electrical separation thereof, is not immediately possible with the aid of the known fixing system, so that further complication of the known mounting system is necessary for this purpose. A further decisive disadvantage of the known field coil is that the power must be supplied via the mounting system or via the bolts, so that series resistors are necessary to balance the current.